The present invention relates to a method of magneto-optical recording and an apparatus therefor, and more particularly to a method for recording magneto-optical information capable of overwriting (writing new data directly on old data) by merely irradiating a radiation beam such as a laser beam without applying an external modulation magnetic field, and an apparatus therefor such as an overwritable magneto-optical disk system.
As an overwriting method in the magneto-optical disk system, a magnetic field modulation system and a light modulation system have been known. In the magnetic field modulation system, an external magnetic field is applied to a magneto-optical recording film while the application area is heated by an irradiation beam energy to reverse magnetization of the magneto-optical recording film so that the overwriting is attained. (For example, JP-A-60-251539, JP-A-60-261051, JP-A-61-22452).
On the other hand, as the overwriting method by the light modulation system, a two-layer magnetic film method (Sato et al. Extended Abstracts (The 34th Spring Meeting, 1987), The Japan Society of Applied Physics and Related Societies, No. 3,28 p-ZL-3, p. 721, and JP-A-62-175948), and a demagnetizing field method Shiek et al., (IEEE trans. Magnetics , Vol. MAG-23, No. 1, January 1987, pp. 171-173) have been reported. In the light modulation overwriting method which uses the demagnetizing field described in the above reference, the overwriting is effected by only the irradiation of a laser beam without applying an external magnetic field. Namely, a recording magnetic domain is formed by a recording laser pulse having a predetermined time duration, and an erasing laser pulse having a shorter time duration than the recording laser pulse is irradiated to the recording magnetic domain to erase the recorded data.
Of the prior art methods described above, the magnetic field modulation overwriting method needs high speed switching of the applied magnetic field. Accordingly, it is difficult to write at a recording frequency above 10 MHz and hence it is not suitable for high speed recording.
On the other hand, in the light modulation overwriting method which uses the two-layer magnetic film, magnetic properties of both magnetic layers such as compensation temperatures and coercive forces must be in a particular narrow range. Accordingly, the media which can be used are limited and the control of the media characteristic is difficult to attain.
In the light modulation overwriting method which uses the demagnetizing field, the overwriting is not attained unless the data is overwritten at the exact position of the pre-written recording magnetic domain. In a self-clocking system used in a conventional optical disk system, a recording clock signal is generated by a frequency-fixed crystal oscillator. Accordingly, if there is an eccentricity in the disk or a variation in a rotating speed, the writing timing may deviate by several microseconds. Thus, it does not assure a sufficient precision required by the overwriting.